I am a Warrior of the Dark...one of the four who stopped the flood of light a thousand years ago.
The Warriors of the Dark are four enigmatic characters. Chosen by their world's crystals, they fought to save the world from the light, which had begun to rampage out of control and threaten to destroy both worlds.
Sounds familiar, doesn't it?
As far as FF3's concepts go, The Warriors of the Dark are one of the more interesting. Both of the prior Final Fantasys operated under a standard "Light good, Dark bad" form of logic, even with the occasional hint of otherwise. III introduces the idea that both Light and Darkness can be bad if they aren't kept in balance, to the point of there being Dark Crystals and Warriors to go along with them. However, the Warriors of Darkness are primarily a background element: The Flood of Light happened centuries ago and details on it are scarce. The Warriors themselves only appear at the end of the game, emerging after the monsters antagonising the Dark Crystals are defeated, giving speeches about the nature of the two worlds and then sacrificing themselves by hurling themselves at the Cloud of Darkness. Even so, the Warriors of the Dark leave a strong impression, and some even speculate on the nature of the Warriors of the Light's future, given how the Warriors of the Dark were sealed in their crystals.
The remake adds to the Warriors of Darkness, albeit in slight and indirect ways much like Xande. The Warriors still remain faceless and their speeches are rewritten to moreso clarify what is going on with the plot in general rather than focusing specifically on the nature of the worlds. Still, there are a few interesting things that they mention; One of them mentions that he and his comrades saved the world "at great cost" while another mentions that a core of darkness used to be surrounded by light but now a core of light is surrounded by darkness.
The unused text strings reveals these comments to be payoffs to the Warriors of the Light's interactions with Doga: That the Warriors of the Dark were said to have died in stopping the flood of light and that the relationship between the earth and the sun and which revolves around which has flipped. Combined, these imply that whatever the Warriors of Darkness did to save the world must have been wild. Maybe someday we'll get a prequel that SE may or may not ruin.
The Final Fantasy I*II*III: Memory of Heroes novelization isn't perfect. In regards to the III portion, even less so: Some events are skipped over, it implies that the Warriors of the Light just popped into existence on Cid's airship and Luneth and Ingus get a disproportionate amount of focus compared to Arc and Refia (which is jarring as their personalities are different compared to the game). Still, it has some interesting elements to it, and none are more interesting than the Warriors of the Dark's portrayal and how it plays into the reworked version of III's endgame.
The Warriors of the Dark make their entrance saving the Warriors of the Light from sinking into the darkness created by the Cloud of Darkness. They reveal that they were chosen to fight against the Cloud of Darkness when the light ran rampant in the past, but lacked the means to truly defeat it and therefore sealed themselves away with it, slowing down its recovery. While Xande's actions allowed it to regain its strength, the Warriors of the Light's arrival means that, if the two parties join together, they can defeat it once and for all. Thus the Warriors of the Light and the Warriors of the Dark jointly fight the Cloud of Darkness and, after a tense battle, manage to defeat it. However, the fate of the Warriors of the Dark is left ambiguous after the Warriors of Light return, leaving it unclear if they died and passed on or if they were sent back to the World of Darkness (if it even exists in this version).
Another interesting wrinkle is that the Warriors of the Dark explicitly use jobs in this version; Their party consists of a Knight, A Sage, A Black Belt and a Ninja. Interestingly, the Warriors of the Light use half of this party by that point (Luneth is a Knight and Ingus is a Ninja; Arc and Refia break the pattern by being a Sage and a Summoner respectively).
The concept of the Warriors of the Dark occasionally echoes throughout the series.
Final Fantasy V, while not featuring the Warriors of the Dark exactly, does feature two sets of Warriors. In this case, it's framed as a generational thing, with the Warriors of Light's predecessors being the Warriors of Dawn. Final Fantasy V does touch on or reinterpret a few ideas of Final Fantasy III, but it's also arguably a stretch.
Final Fantasy Dimensions is an episodic mobile game that takes place in a single world that gets split into two: a World of Light and a World of Darkness. As this implies the game features both Warriors of Light and Warriors of Darkness and constantly switches between the two until the end of the game. Dimensions' take overall is something of a cross between Final Fantasy III, Final Fantasy V and the Four Heroes of Light of all things: The two worlds split from one due to a crystal being shattered, the worlds split into a light and dark world with the Warriors having both shared and exclusive-to-their-team jobs and said lattermost jobs are gained one at a time rather than all at once.
Final Fantasy XIV also uses the concepts of the Warriors of Darkness, albeit in a different way compared to how III did it, due to the nature of the Warriors in question.